trace: add benchmark for SpanFromHeader
trace: add benchmark for SpanFromHeader

$ go test -bench=Benchmark -benchmem
BenchmarkSpanFromHeader-8   	 5000000	       268 ns/op	     512 B/op	       2 allocs/op

Change-Id: I4a7367affbd93467feafc33b4b547f3e9b6619a8
Reviewed-on: https://code-review.googlesource.com/18010
Reviewed-by: Ross Light <dfccc06f14414bff5a59be7fc90abf4404b47fc8@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go